1. Implement Trie (Prefix Tree)

A trie (pronounced as “try”) or prefix tree is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. There are various applications of this data structure, such as autocomplete and spellchecker.

Implement the Trie class:

• Trie() Initializes the trie object.
• void insert(String word) Inserts the string word into the trie.
• boolean search(String word) Returns true if the string word is in the trie (i.e., was inserted before), and false otherwise.
• boolean startsWith(String prefix) Returns true if there is a previously inserted string word that has the prefix prefix, and false otherwise.

Example 1:
Input
["Trie", "insert", "search", "search", "startsWith", "insert", "search"]
[[], ["apple"], ["apple"], ["app"], ["app"], ["app"], ["app"]]
Output
[null, null, true, false, true, null, true]

Explanation
Trie trie = new Trie();
trie.insert("apple");
trie.search("apple"); // return True
trie.search("app"); // return False
trie.startsWith("app"); // return True
trie.insert("app");
trie.search("app"); // return True

Constraints:

• 1 <= word.length, prefix.length <= 2000
• word and prefix consist only of lowercase English letters.
• At most 3 * 104 calls in total will be made to insert, search, and startsWith.

Approach - Hash Map

• It is like tree but think of like this we has a hash map with character and what the next node would be in the value then we have an end variable to indicate end of a word
• Time complexity: O(n) for each function call.
• Space complexity: O(t)

Where n is the length of the string and t is the total number of TrieNodes created in the Trie.

class TrieNode {
    Map<Character,TrieNode> child = new HashMap<>();
    boolean end = false;
}
 
class Trie {
    private TrieNode root;
    public Trie() {
        root = new TrieNode();
    }
    
    public void insert(String word) {
        TrieNode current = root;
        for (char c: word.toCharArray()) {
            current.child.putIfAbsent(c,new TrieNode());
            current = current.child.get(c);
        }
        current.end = true;
    }
    
    public boolean search(String word) {
        TrieNode current = root;
        for (char c: word.toCharArray()) {
            if (!current.child.containsKey(c))
                return false;
            current = current.child.get(c);    
        }
        return current.end;
    }
    
    public boolean startsWith(String prefix) {
        TrieNode current = root;
        for (char c: prefix.toCharArray()) {
            if (!current.child.containsKey(c))
                return false;
            current = current.child.get(c);    
        }
        return true;
    }
}
 
/**
 * Your Trie object will be instantiated and called as such:
 * Trie obj = new Trie();
 * obj.insert(word);
 * boolean param_2 = obj.search(word);
 * boolean param_3 = obj.startsWith(prefix);
 */